1. Field of the Invention
The present invention relates to electronic binoculars, and in particular to electronic binoculars having a control function in which a dynamic range of an imaging device is substantially expanded when photographing an observed image containing parts where a luminance difference is great.
2. Description of the Related Art
Conventionally, electronic binoculars have been proposed that are capable of recording image data in a storage medium, and further, have the advantage of night vision. The electronic binoculars are generally provided with an imaging unit, ocular units, and an image-signal processing unit. The imaging unit has an optical system such as a photographing optical system, and an imaging device for converting an optical image, obtained by the optical system, to an electric signal, which is converted to an image signal by the image-signal processing unit. The image signal is transmitted to the ocular units, so that an image is indicated by the ocular units.
An imaging device, such as a CCD, converts an optical image to an electric signal, which is picked up by the image-signal processing unit at a regular imaging interval, and converted to an image signal.
When an observed image contains parts in which a luminance difference is great, the imaging device needs to have a great dynamic range in order to properly photograph the image. If the imaging time is too short, the amount of light entering the imaging device is not sufficient, so that a loss of detail occurs in dark areas, in which a dark part is not photographed while a bright part is clearly indicated. Conversely, if the imaging time is too long, the amount of light entering the imaging device will exceed the saturation signal amount of a photodiode provided in the imaging device, so that a washing out occurs in bright areas, in which a bright part is saturated while a dark part is clearly indicated.
The loss of detail and washing out effect can be reduced by a dynamic range expansion, in which an electric signal, obtained by using a short imaging time operation, and an electric signal, obtained by using a long imaging time operation, are superimposed, so that the dynamic range is substantially expanded in accordance with the ratio of the short and long imaging times.
Accordingly, in conventional dynamic range expansion, it is necessary to have at least two images which are obtained by using different imaging times, and correspond to a common observed object. Thus, after a first exposure is completed, a second exposure is performed. Therefore, this photographing method is disadvantageous if there is movement of the observed object. Further, it is disadvantageous if there is a camera shake even if the observed object is still. Due to this, it is difficult to apply conventional dynamic range expansion to binoculars having a telephotographic optical system.